T. W. McDaniel

Temperature distributions produced in an N‐layer film structure by static or scanning laser or electron beam with application to magneto‐optical media

The classic problem of heat flow in multilayer film structures has been revisited from the perspective of engineering applications for fields such as optical storage media design, laser annealing of semiconductor materials, electron beam lithography, and ion implantation. A compact recursive structure N‐layer Green’s function is developed from the coupled partial differential equations of thermal conduction. Temperature profiles are calculated for the absorption of normally incident continuous and pulsed‐Gaussian‐beam irradiation, on both static and moving media, with variable absorption across the source layer. General beam, amplitude‐time dependencies can be calculated with this formalism; however, only simple rectangular pulses are treated in the text. General N‐layer solutions are developed, and compact, recursive‐integral formulas, whose evaluation is enhanced with modern computer languages such as c and apl, are derived. In addition, we offer a clear physical interpretation of the results expressed ...

Magneto‐optical Kerr effect and perpendicular magnetic anisotropy of evaporated and sputtered Co/Pt multilayer structures

Thin and ultrathin Co/Pt multilayered structures have been prepared on glass substrates by electron‐beam evaporation at room temperature and by sputtering at various substrate temperatures and sputtering pressures. Perpendicular magnetic anisotropy was found in samples with Co/Pt bilayer thicknesses near 3 A/10 A and total thicknesses of the layer stack of no greater than 300 A. X‐ray diffraction was performed on the samples to determine layer spacing and integrity, and possible crystallinity of films. Crystalline structures in the interface between the Co and Pt layers were found and identified. The effects of sputtering parameters, such as pressure and substrate temperature, on the magneto‐optical Kerr effect were studied. The two deposition methods, electron‐beam evaporation and sputtering, resulted in different magneto‐optical properties in samples with the same nominal layer structures. We have also investigated optical properties (reflectance, index of refraction, and extinction coefficient) of these materials using ellipsometry.

Numerical simulation of thermomagnetic writing in RE-TM films

In the development of high-performance magneto-optic (MO) media for data storage, it is important to understand the processes by which reversed magnetic domains are formed in thermomagnetic writing. Thin films of amorphous rare earth-transition metal (RE-TM) alloys deposited by sputtering on a nonmagnetic substrate have become a popular choice for an erasable storage medium. We have developed a simplified micromagnetics simulation of the behavior of RE-TM films that produces an equilibrium configuration of film magnetization for a specified thermal profile and applied magnetic field. This Monte Carlo-type model is useful for assessing the size, shape, and stability of written marks in MO recording. In addition, the simulation provides estimates of the minimum field strength required for nucleation and growth of reversed domains, and hence it predicts film coercivity at any temperature. An equilibrium state is determined by magnetic energy minimization. Magnetization dynamics are not incorporated into the model, but instead, a quasi-static approach to equilibrium is simulated. Spatial resolution can be arbitrarily small, and discretization down to about 20 nm can be reasonably accommodated. Readout of written domains has been incorporated as an additional feature of this model.

Optical and magneto-optical characterization of TbFeCo thin films in trilayer structures

A series of TbFeCo films ranging in thickness from 100 to 800 A have been deposited in trilayer structures on silicon wafer substrates, with Si3N4 being employed as the dielectric material. These films have been characterized both optically and magneto‐optically by variable angle of incidence spectroscopic ellipsometry, normal angle of incidence reflectometry, and normal angle of incidence Kerr spectroscopy. From these measurements, the optical constants n and k have been determined for the TbFeCo films, as well as the magneto‐optical constants Q1 and Q2. Results are presented that demonstrate the lack of dependence of these constants on the thickness of the TbFeCo film, and which can be used for calculating the expected optical and magneto‐optical response of any multilayer structure containing similar TbFeCo films.

Sputtering pressure effect on microstructure of surface and interface, and on coercivity of Co/Pt multilayers

Thin Co/Pt multilayers were prepared on Si and glass substrates by sputtering with Ar pressures ranging from 2.5 to 15 mTorr. The bilayer structure of the samples was Co(3 A)/Pt(15 A)×17, and all samples had the easy axis of magnetization perpendicular to the sample surface as determined with a SQUID magnetometer. All samples retained the layered structure, as revealed by low‐angle x‐ray diffraction. In addition, diffraction peaks due to the formation of Co‐Pt compounds (presumably at the interfaces between Co and Pt) were identified. The coercivity of samples changed from about 400 Oe for films deposited at low Ar sputtering pressure (2.5 mTorr) to as high as 2300 Oe for films deposited at high Ar pressure (15 mTorr). Ellipsometry and atomic force microscopy were used to study surface roughness and microstructure of samples prepared at different sputtering pressures.

Design and characterization of a quadrilayer magneto-optic disk

The use of quadrilayer film structures can maximize the magnetooptic (MO) effect in reflected light used for readout in optical data storage. A computer program and a multifilm optical model have been utilized to search for optimized quadrilayer structures based on various figures of merit (FOMs), including differential MO signal, shot-noise-limited SNR, and SNR expressions arising from other noise sources. Selection among various FOMs allows tradeoffs relative to the magnitude of the reflectance or thermal efficiency in writing. Disks were fabricated on the basis of the simulation design rules, and experiments were conducted to assess the recording performance. The authors report on the optimization search used in the design program and the formation of the associated FOMs, and they present data on signal and noise to illustrate the value of the model. >

Simulation of bit jitter in magneto‐optic recording

The interplay of several writing and reading phenomena that influence bit‐position accuracy has been analyzed with a computer model of the magneto‐optic (MO) recording process. A micromagnetics simulation determines a minimum energy magnetization state of a MO storage film following local laser heating in an externally applied bias magnetic field. Bit jitter is assessed by measuring the length, width, and position of written domains, and by analyzing the modeled readout signal and its derivative. The influence of variation in the magnetization, anisotropy, and exchange in an amorphous rare‐earth–transition‐metal film on the position and size of recorded marks has been simulated by specifying random variations of a film’s magnetic properties in the thermomagnetic writing step. Variations in medium thermal sensitivity and the reading laser spot size were also considered. Histograms showing the effects of write‐read imperfections on bit‐position accuracy in MO recording were developed. Comparisons with exper...

MO media noise studies by readout beam scanning

The spectral distribution of media noise in magnetooptic (MO) disks is discussed. The spatial impulse response of the readback system has been computed by scanning a spot across a straight-line MO feature transverse to an otherwise featureless track. An alternative calculation involved scanning the beam over 2-D sinusoidal and checkerboard MO patterns of variable wavelength. The impulse response was combined with the measured spectral content of wide, noisy, self-demagnetized written tracks on MO media to infer characteristics of the spatial frequency distribution of media noise. Particular emphasis was placed on determining the extent to which the media spatial frequency distribution is white, or equivalently, whether particular correlation lengths are dominant. It was found that media noise induced in thermomagnetic writing or periodic data patterns has a spectral shape that is independent of the frequency of the written pattern, but with a noise power that is proportional to the pattern frequency. >

Thermal mark characterization on static magneto-optical media

The authors explore the thermal behavior of multilayer magnetooptic (MO) storage media in thermomagnetic recording. An attempt has been made to develop a variety of useful models for heat transfer in these disks that can be readily interpreted for ongoing media design activity. Experimental techniques have been devised that can probe the spatial and temporal distribution of the thermal field in the MO storage film. An important feature of these experimental methods is that they use conventional recording hardware for testing recording performance. The experimental work focuses on static, zero-velocity recording experiments in order to clarify the methodology of using thermomagnetic recording process as a high-speed, high-resolution thermometer for the MO film. It is found that each of the models (finite element and method of images) provides an accurate description of experimental measurements of the thermal field in typical MO disk structures. >

Magnetization reversal dynamics in magneto‐optic media (invited)

Dynamics of magnetization reversal in thin films of amorphous rare earth‐transition metal alloys are investigated. Using computer simulations for a two‐dimensional square lattice of dipoles with nearest‐neighbor exchange interaction and random axis anisotropy, the Landau–Lifshitz–Gilbert equation is numerically integrated to yield the static wall structure and its motion under an applied field. The simulated results show excellent agreement with theory in the special cases where analytic solutions exist.